1. Field of the Invention
The present invention relates to a method of forming a toner image on an image transfer sheet, comprising the steps of developing a latent electrostatic image formed on an image bearing member to a toner image using a developer comprising a toner which comprises a coloring agent having a chromatic color and being capable of retaining the chromatic color when the toner is sintered, and transferring the toner image from the image bearing member to an image transfer sheet, thereby obtaining a toner image bearing transfer sheet. The present invention also relates to a method of fire fixing an image on a heat resistant solid surface using the above obtained toner image bearing transfer sheet. In addition, the present invention also relates to the above-mentioned developer and toner image bearing transfer sheet for use with the fire fixing method.
2. Discussion of Background
To form an image or pattern on the surface of a heat-resistant solid such as ceramics, a desired image or pattern is directly written on the surface of the heat-resistant solid with a brush using a coloring material comprising an inorganic pigment and a glaze, and thereafter the image bearing heat-resistant solid is subjected to firing at 750 to 1,300.degree.C.
According to the above-mentioned conventional firing method, some components for use in the coloring material are sintered and the glaze is melted in the course of the firing step, and the inorganic pigment is fixed to the heat-resistant solid surface by the aid of the glaze when cooled to room temperature after sintered. Thus, there remains on the heat-resistant solid surface the image first formed thereon in handwriting.
The above-mentioned firing method has the drawback that the same image or pattern must be formed in handwriting on a plurality of ceramic products only by a skilled craftsman no matter how simple the image or pattern may be.
To produce large quantities of ceramic products which bear the same image or pattern thereon, it is therefore proposed that a desired image be first formed on a transfer sheet by screen printing process. The thus formed image-bearing portion is separated from the transfer sheet and attached to the surface of each ceramic product, and then sintered so as to fix the image to the ceramic product.
The above-mentioned fire fixing method using the screen printing process is proposed, for instance, as disclosed in Japanese Laid-Open Patent Application 49-35407. By this kind of fire fixing method, a colored image is formed on a ceramic product in accordance with the following procedure:
A transfer sheet comprises a support and a water-soluble paste layer formed thereon. An image is printed by the screen printing process on the above-mentioned water-soluble paste layer of the transfer sheet with an ink comprising an inorganic pigment, and a water-insoluble resin film layer comprising a vinyl or cellulose-based resin is provided on the printed ink image.
The image-bearing transfer sheet is immersed into water. The water-soluble paste layer is dissolved in water and the support peels off, whereby there remains an ink-image bearing film member.
The ink-image bearing film member thus obtained is applied to the heat-resistant solid surface such as a ceramic plate, and sintered, whereby the sintered image is fixed to the ceramic product.
According to this method, a large number of ceramic products that bear the same image thereon can be obtained in such a manner that the same image is printed on many transfer sheets by the screen printing, each of the thus prepared ink-image bearing transfer sheets is attached to the surface of the ceramic product, and the image-bearing ceramic products are sintered.
In the screen printing, however, making of a printing plate for forming the ink image includes many steps and requires a great deal of time and labor. The unit cost is necessarily increased, in particular, when various kinds of items are produced, with each item having the same image thereon. Further, the screen printing process is apt to make worse the working conditions due to an air pollution problem caused by the evaporation of organic solvents from the ink employed in the screen printing.
To solve the problems caused by the screen printing process, there is proposed a method of forming a toner image using the electrophotographic process on the same transfer sheet as employed in the above. To be more specific, an image is formed on the transfer sheet using an inorganic toner which comprises an organic polymer, composite particles of an inorganic pigment and glass powder, a binder resin and a pigment for ceramics. In the same manner as stated in the above, the toner image bearing film member is separated from the support of the transfer sheet and attached to the surface of the ceramic product. Then, the toner image thus attached to the ceramic product is sintered, so that the toner image can be easily fixed to the surface of the ceramic product.
The aforementioned method is proposed, for example, in Japanese Laid-Open Patent Applications 4-135798, 7-199540, 7-214890, 7-228037, 7-300382, 8-104050, 8-11496 and 8-119668.
According to the above-mentioned proposed method using the electrophotographic process, the image formation step can be drastically simplified as compared with that by the screen printing process, and various kinds of items, each item having the same image thereon, can be easily manufactured even though the production of each item is on a small-scale.
However, when the toner image prepared by the electrophotographic process is applied to the surface of the ceramic product and sintered, there are the problems that non-transferred spots appear in the sintered toner image and the sintered image is impaired by uneven firing.